This invention relates to a compact slide clearing apparatus for use with phase contrast microscopy.
More particularly, this invention relates to an apparatus for clearing a filter, such as a filter containing asbestos fibers or other particulates. The apparatus is used to clear a cellulose ester or nitrate membrane filter with acetone vapors. In the apparatus a small amount of liquid acetone is rapidly vaporized and ducted to the filter to provide even and rapid clearing of the filter to render it translucent during subsequent optical microscopy.
The apparatus of the invention is employed as an element in a method which is particularly useful for analyzing filter sample concentrations for asbestos fibers. The analyzing method involves chemically collapsing the cellulose ester or nitrate filter with a stream of acetone vapor to provide a clear background against which to count the individual fibers with an optical microscope.
To clear an individual filter sample about 0.2 ml of liquid acetone is injected into the apparatus. The liquid flows downwardly through the aparatus wherein it is vaporized. The vapor is ejected through a nozzle and is condensed on the filter which is located on a glass slide. The condensation is rapid. The entire injection and clearing process takes no more than about 5 onds.
A glass slide carrying a one-fourth pie-shaped segment of a 25 mm filter can be positioned on a galss slide holder in the apparatus. The apparatus is preheated and 0.2 ml liquid acetone can be loaded into the apparatus by pipette or syringe. The amount of acetone will depend on the size of the filter element with the amount increasing with an increase in the size of the filter element. The apparatus will vaporize the acetone at a temperature in the range of about 155.degree. to 165.degree. F. and then eject the acetone vapor over the slide. The entire clearing operation will occur within about 5 seconds following which the slide is immediately removed. If the slide is subjected to the heat of the apparatus for a longer time period it can curl up and the cleared filter can become distorted. The amount of acetone used should be kept to a minimum because acetone can constitute a fire hazard.
If the apparatus is operated at temperatures above the range 155.degree. to 165.degree. F. the acetone may boil violently, causing liquid droplets to reach the surface of the filter. Also, at higher temperatures the acetone may tend to decompose. At lower operating temperatures, the acetone may stay on the filter and slide for an unduly long time period allowing the surface tension of the liquid to draw dissolved filter material and particles away from the edge of the filter.
In the analyzing method, the cleared filter on the slide is then covered with triacetin and a cover slip. About a ten minute wait allows maximum clearing of the filter surface. The slide is then ready for examination with an optical microscope.